nfp: flower: add GRE decap classification support

Extend the existing tunnel matching support to include GRE decap
classification. Specifically matching existing tunnel fields for
NVGRE (GRE with protocol field set to TEB).

Signed-off-by: Pieter Jansen van Vuuren <pieter.jansenvanvuuren@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: John Hurley <john.hurley@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>